Method and magnetic resonance tomography apparatus for correcting changes in the basic magnetic field

Abstract

In a method for calculating a one-dimensional map of the base magnetic field B0 and subsequent correction, based on this map, of image artifacts due to B0 field changes, in the examination of a subject with a magnetic resonance tomography apparatus, data for a slice of the subject to be examined are obtained with a sequence of a fast MRT imaging method that includes at least three phase correction scans and measurement signals of the respective phase correction scans as well as of the slice are obtained, the phase difference of the data points (respectively pertaining to one another) of two phase correction scans, in the form of the acquired measurement signals of the respective phase correction scans, are calculated point-by-point, the average phase difference between the phase correction scans is evaluated, and the frequency offset between the actual resonance frequency relative to the adjusted resonance frequency is calculated based on the average phase difference and the echo time difference between the phase correction scans used, a B0 field map is calculated dependent on the frequency offset and, the measurement data for the slice are corrected using the calculated B0 field map.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally concerns magnetic resonance tomography (MRT) as employed in medicine for the examination of patients. The present invention is in particular concerned with a method for determination of the B0 (basic magnetic) field, in particular given the use of fast MRT imaging methods such as, for example, TSE, EPI, SSEPI. A projection obtained of the B0 field (B0 field map) of the acquired slice enables the correction of image artifacts arising due to changes in the B0 field. 2. Description of the Prior Art MRT is based on the physical phenomenon of nuclear magnetic resonance and has been successfully used as an imaging method in medicine and biophysics for over 15 years. In this examination method, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms of the subject, which were previously oriented randomly, thereby align. Radio-frequency energy can now excite these “o...

AI Technical Summary

Benefits of technology

An object of the present invention is to provide a method with which, in a simple manner and for every MRT apparatus, in particular for functional, contrast agent-aided and diffusion measurement methods, during the measurement the B0 field can be quantified for each slice absolutely or relative to the beginning of the measurement, and on this basis a correction can be effected only in the image plane or in k-space.

Problems solved by technology

In addition to this, interferences of local field inhomogeneities increase with increasing readout time.

Given the combination of EPI with contrast agent-supported methods (perfusion imaging), however, these effects can be corrected only with difficulty because the successive images are very different due to the contrast agent bolus flowing therethrough.

This can lead to errors or given a low residual signal in the images, to outright impossibility of the image analysis.

A similar problem occurs in diffusion imaging.

The data of a single acquisition are unusable for a correction due to the high noise portion.

If, for example, the B0 field now changes during the diffusion measurement, the individual acquisitions are displaced relative to one another, and the resulting image quality can be impaired.

Moreover, not only are temporal changes of the B0 field problematic, but also the variation of the absolute value of B0 across the sample or the patient to be examined.

In the case of echo planar imaging, this in turn leads to an apparent displacement of the measurement subject, meaning the planned or calculated image position (slice position) is typically incorrect by a few millimeters (up to centimeters).

Problems with the interpretation and further use of the obtained images—for example given overlay with other measurement results for further planning of the examination and / or therapy—can result from this.

In the case of imaging methods with significant contrast changes (such as, for example, diffusion imaging, perfusion imaging), however, this method frequently cannot be successfully used (given contrast changes, a comparison of the acquired data is increasingly more difficult).

The method for correction proposed by Durand et al has also proven in practice to be only of limited capability.

Due to movements (for example, brain pulsation), diffusion imaging a priori leads to unknown displacements of the k-space data, such that a comparison of identical k-space lines is problematic.

Moreover, in the diffusion coding, additional phase differences between the individual acquisitions of the comparison proposed by Durand et al of central k-space can be hampered or even can be completely impossible to devise.

Finally, the received signal of individual acquisitions is increasingly severely noisy, in particular at higher diffusion coding strengths, such that no sufficiently precise phase information is available.

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